The Reaction Vehicle

If the rifle was now attached to a lightweight cart (Figure 14)
and fired, it would be set in motion by the force of the recoil.
If the rifle was fired continually and rapidly, approximately
similar to a machine gun, then the cart would be accelerated,
and could also climb, etc. This would be a vehicle with reaction
propulsion, not the most perfect, however. The continual movement
of a vehicle of this type takes place as a result of the fact
that it continually accelerates parts of its own mass (the
projectiles
in the previous example) opposite to the direction of motion and
is repelled by these accelerated parts of mass.

Figure 14. A primitive vehicle with reaction propulsion: The cart
is moved by continuous firing of a rifle, as a result of the
"reaction"
generated thereby.

It is clear as a result that this type of propulsion will then
be useful when the vehicle is in empty space and its environment
has neither air nor something else available by which a repulsion
would be possible. Indeed, the propulsion by recoil will only
then be able to develop its greatest efficiency because all
external
resistances disappear.

For the engineering design of a vehicle of this type, we must
now strive to ensure that for generating a specific

propulsive force, on the one hand, as little mass as possible
must be expelled and, on the other hand, that its expulsion
proceeds
in as simple and operationally safe way, as possible.

To satisfy the first requirement, it is basically necessary that
the velocity of expulsion be as large as possible. In accordance
with what has already been stated, this can be easily understood
even without mathematical substantiation, solely through intuition:
for the greater the velocity with which I push an object away
from me, the greater the force I have to apply against it; in
accordance with what has already been stated, then the greater
the opposite force will be that reacts on me as a result; this
is the reaction produced by the expulsion of precisely this mass.

Furthermore, it is not necessary that larger parts of mass are
expelled over longer time intervals, but rather that masses as
small as possible are expelled in an uninterrupted sequence. Why
this also contributes to keeping the masses to be expelled as
low as possible, follows from mathematical studies that will not
be used here, however. As can be easily understood, the latter
must be required in the interest of operational safety, because
the propulsive thrust would otherwise occur in jolts, something
that would be damaging to the vehicle and its contents. Only a
propulsive force acting as smoothly as possible is useful from
a practical standpoint.